code: def __len__(self):
      return self._len
  
file_path: fairseq/fairseq/data/nested_dictionary_dataset.py
imports: from collections import OrderedDict
  import torch
  from torch.utils.data.dataloader import default_collate
  from .None import FairseqDataset
label_desc: Use image node_id 3 for calling the NestedDictionaryDataset obj's underlying member method code with example usage: obj.__len__() and returns: self